1. Field of the Invention
The present invention relates to a light emitting device that uses an element having an anode, a cathode, and a film containing an organic compound in which EL (electroluminescence, luminescence which develops by the application of an electric field) is obtained (The film is hereafter referred to as an organic EL layer, and the element is hereafter referred to as an organic EL element.). The materials used in the organic EL layer, and the main cathode materials, show a remarkable proclivity to degrade due to moisture. Therefore, a drying agent is included during normal sealing. The present invention in particular relates to a light emitting device in which a drying agent made from a compound capable of chemically absorbing moisture and maintaining its solid state after moisture absorption is formed as a porous body. Note that, within this specification, the term light emitting device indicates an image display device or a light emitting device which uses organic EL elements as light emitting elements. Further, modules in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to the organic EL elements, modules in which a printed wiring substrate is formed at the tip of the TAB tape or TCP, and modules in which an IC is directly mounted to the organic EL elements by a COG (chip on glass) method are all contained in the category of light emitting devices.
2. Description of the Related Art
Organic EL elements are elements which emit light by the application of an electric field. The light emitting mechanism is one in which electrons injected from a cathode and holes injected from an anode recombine at light emitting centers within an organic EL layer, forming excited state molecules (hereafter referred to as molecular excitons) due to the application of an electric field between electrodes sandwiching the organic EL layer. The molecular excitons radiate energy when returning to a base state, thereby emitting light.
The organic EL layer is formed having a film thickness of less than 1 μm in normal organic EL elements. Further, the organic EL elements themselves are self light emitting elements, and therefore a backlight, such as that used in conventional liquid crystal displays, is not necessary. Thus, the organic EL elements have a large advantage that they can be manufactured very thinly and with very light weight.
The amount of time from the injection of a carrier until recombination in an organic EL layer having a thickness on the order of 100 to 200 nm, for example, is on the order of several tens of nanoseconds when considering the carrier mobility of the organic EL layer, and even if the process from recombination until the emission of light is included, this leads to the light emission within one microsecond. The organic EL elements therefore have an extremely fast response speed.
In addition, the organic EL elements are carrier injection light emitting elements, and therefore can be driven by a DC voltage. It is possible to have a driving voltage on the order or several volts by employing a method of selecting an electrode material that makes the carrier injection barrier smaller, a method of introducing a hetero structure (lamination structure) or the like. (Reference 1: Tang, C. W., and VanSlyke, S. A., “Organic Electroluminescent Diodes”, Appl. Phys. Lett., 51, No. 12, pp. 913–915 (1987)). Low voltage DC drive is achieved by the authors of Reference 1 by using an Mg:Ag alloy as a cathode, and employing a hetero structure in which an aromatic diamine compound and an aluminum chelate complex are laminated.
As explained above, the organic EL elements are drawing attention as flat panel display elements in the next generation due to their thinness, light weight, high speed response, and their ability to employ low voltage DC drive. Further, they are self light emitting and thus the field of view is wide, providing relative ease in view. They are considered effective as elements used in the display screens of portable devices.
Structures of the organic EL elements are mainly such that a transparent electrode (for example, ITO) as an anode, organic EL layers, and a cathode material are laminated in order on a glass substrate or a plastic substrate (hereafter referred to simply as a substrate), and light is extracted from the substrate side. Further, not only this structure, but also a number of other structures have been considered in recent years because light is able to be extracted provided that one of electrodes is transparent. For example, there are methods such as one in which light is extracted from the side opposing the substrate by using a transparent electrode in the cathode side.
In order to maintain the elements within an airtight environment, normally an opposing substrate is joined to the substrate and sealed with the organic EL elements manufactured as above. Namely, the organic EL elements are formed within a container that is cut off from the atmosphere. Sealing is also performed similarly for light emitting devices using organic EL elements.
One goal of sealing is protection from mechanical factors (such as pressure and shock), but there is another very fundamental and important goal. That goal is protection from chemical factors (moisture and oxygen). Materials used in organic EL layers, and cathode materials mainly using metals with small work function (active, in other words) are easy to react with moisture and oxygen, which easily brings about element degradation.
In particular, non-light emitting portions normally generated with a circular shape (hereafter referred to as dark spots) grow under the presence of moisture, and it has been reported that this growth can be well controlled under a dry gas. (Reference 2: Kawaharada, M., Ooishi, M., Saito, T., and Hasegawa, E., Synth. Metals, 91(1997), p. 113). This can be thought of as one example showing that moisture greatly influences the degradation of the elements.
Sealants used for joining substrates and opposing substrates during sealing are generally light setting resins. If thermal setting resins are used, then the organic EL layers are also heated during setting, and changes in film quality and degradation of the material itself develop. However, even if the EL elements are cut off from the outside atmosphere by using the light setting resins, it is difficult to have complete cut off from moisture and oxygen. For example, a large amount of dark spots develop with organic EL elements sealed by a UV setting resin when they are held under high temperature and high humidity, thereby promoting the element degradation.
Thus a method of introducing a drying agent capable of absorbing moisture (hereafter referred to as hygroscopic) during sealing is generally used nowadays. It is thought that there is no effect on oxygen, but it is at least possible to absorb moisture which has penetrated to the inside of the container cut off from the atmosphere, and therefore the element degradation due to moisture can be suppressed to a certain extent.
For example, elements have been disclosed in which a protective case is formed around the organic EL elements, and a fine powdered solid dehydrating agent fills the protective case (Reference 3: Japanese Patent Application Laid-open No. Hei 6-176867). Zeolites, active alumina, silica gels, calcium oxide, and the like are given as examples of the fine powdered solid dehydrating agents in Reference 3.
However, if drying agents which physically absorb moisture are used, such as the zeolites and silica gels shown in Reference 3, then the moisture which has been absorbed is emitted due to Joule heat generated when the organic EL elements emit light, and there is a danger that the growth of dark spots cannot be sufficiently controlled.
Organic EL elements using a chemical compound capable of chemically absorbing moisture, and maintaining its solid state even with adsorbed moisture, as a drying agent has also been disclosed (Reference 4: Japanese Patent Application Laid-open No. Hei 9-148066). Once absorbed, moisture is not easily emitted due to heat when this type of drying agent is used, and liquification due to absorbed moisture does not cause any bad influence on the elements. Alkaline metal oxides, alkaline-earth metal oxides, sulfates, metal halogen compounds, perchlorates, and organics are given as examples of the drying agents in Reference 4.
Note that the drying agents like those stated above are mostly used in a finely powdered state. It is possible to use them in a bulk state, but the surface area becomes relatively larger for the case of fine powders at the same volume, and therefore a large hygroscopic effect can be obtained with a small amount.
A substrate on which fine wirings and the like are implemented is used for a process of manufacturing organic EL elements, and therefore manufacturing is performed within a clean room like that utilized for normal semiconductor manufacturing processes. The purity of the materials used in the organic EL layers is also thought to influence the element properties, and therefore it is essential to perform work within the clean room.
However, if fine powders as drying agents, in particular fine powders of materials containing alkaline metals or alkaline-earth metals are taken into the clean room, they may greatly influence the cleanliness within the clean room.
One reason is the size of the fine powders. Drying agent fine powders are normally on the order of several μm to several tens of μm, and therefore are of a size which is sufficient to cause phenomena such as electrical shorts between adjacent regions at which wirings must be separated, and conversely, separation in regions at which wirings must be electrically connected. Namely, the fine powders cause damage to wiring pattern formation. This is referred to in particular as particle contamination.
Another reason is chemical contamination due to alkaline metal ions and alkaline-earth metal ions when fine powders of alkaline metal oxides and alkaline-earth metal oxides are used as drying agents. Alkaline metal ions generated due to the fine powders cause phenomena such as an increase in the speed of oxidation, for example, and are a cause of defective parts. (Reference 5: Hattori, T., ed., Silicon Wafer Surface Cleaning Technology, (Realize publ.), p. 29.)
Direct contamination, of course, can be prevented provided that the fine powder drying agent hermetically sealed in a can or the like is taken into the clean room, and that the can or the like is then opened at the location for performing sealing. However, it is difficult to completely prevent indirect contamination (cross contamination) through the substrate when the substrate enters and leaves the location for performing sealing work with the fine powder drying agent. In particular, fine powders are easily stirred up into the air, and are easily adsorbed, thereby making cross contamination difficult to be prevented.
It is therefore preferable, from a processing perspective, to use the drying agent in a bulk state or in a film state, so that particle contamination, and chemical contamination deriving from the particles, do not occur. Reference 4 refers to methods of forming drying agents using techniques such as vacuum evaporation, sputtering, and spin coating.
However, the surface area contacting air containing moisture is extremely limited with a normal bulk or film state drying agent (only the surface area of the bulk or the film), and this therefore becomes a problem in that the drying agent does not show sufficient performance in its hygroscopic ability. In other words, there is a danger that simple film formation of the drying agent by using vacuum evaporation, sputtering, spin coating or the like cannot demonstrate sufficient hygroscopic ability. As a consequence, drying agents are currently used in a finely powdered state for actual processing, which needs substantial care.